The present invention relates generally to industrial rolls, and more particularly to supercalender rolls having bone-hard surfaces.
Calendering is the process of passing a sheet material through rolls or plates to impart a smooth, glossy appearance to the sheet material. This process can be enhanced through a xe2x80x9csupercalenderingxe2x80x9d process, in which the sheet material is exposed to heat in addition to the pressure applied by the rolls or plates. Supercalendering is particularly prevalent in the production of SC grade paper (such as that typically used for printing and writing) that often requires a smooth, high density, glossy surface and a uniform caliper.
Because of the demands of the supercalendering process, a supercalender roll should have a xe2x80x9cbone-hardxe2x80x9d calendering surface. The term xe2x80x9cbone-hardxe2x80x9d is generally understood to mean that the surface has an elastic modulus of at least 200,000 psi and a Shore D hardness rating of at least 80. Of course, a supercalender roll should also be constructed of materials that enable it to withstand the extreme pressure, heat and moisture encountered in the supercalendering process.
One type of supercalender roll that has been used historically is the so-called xe2x80x9cfilled roll,xe2x80x9d which is formed of very tightly pressed paper, cotton, or similar natural or synthetic fiber material (such as Kevlar(copyright), Nomex(copyright) or rayon). In some embodiments, annular disks of the fibrous material are stacked on a central shaft and pressed together very tightly by pressure plates located on the ends of the shaft. These disks typically form a layer that extends radially outwardly from the shaft between about 5 and 10 inches. The pressure applied to the disks by the pressure plates is generally sufficient to render the surface of the fibrous material xe2x80x9cbone-hard.xe2x80x9d Exemplary filled rolls are described in U.S. Pat. No. 4,283,821 to Paakkunainen and U.S. Pat. No. 4,475,275 to Edwards.
A filled roll can provide a very light, strong and hard roll, but one that is quite prone to dents or marks on its surface. Of course, such dents or marks can adversely impact the surface of the roll, which may render it unsuitable for a process where surface consistency is important, such as papermaking. One attempt to address this shortcoming involves the inclusion of a polymer cover over a filled roll; one example of this construction is described in U.S. Pat. No. 3,711,913 to Galeone et al. However, many filled rolls having polymer covers have proven unsuitable in that bonding between the cover and the fibrous portion of the roll can be inconsistent, resulting in delamination of the cover. Also, typically the cover is unable to prevent the fibrous portion of the roll from denting under impact. When this occurs, the dented fibrous portion can separate from the cover such that the localized dented areas no longer directly support the cover. As a result, the unsupported areas of the cover can fatigue and ultimately fail under load.
As an alternative, some bone hard supercalender rolls are constructed of an epoxy matrix reinforced with glass fiber and other filler materials, such as organic, carbon or other ceramic fibers. The epoxy matrix is typically applied as a layer approximately 0.4-1.5 inches in thickness over a hollow metal core. Although such epoxy-coated rolls are generally more durable and consistent in operation than are filled rolls, this variety of supercalender roll can be quite expensive to a paper producer because of the costs to purchase all new metal cores.
In view of the foregoing, it is an object of the present invention to provide a supercalender roll with a durable supercalendering surface that is not prone to dents, nicks, and other surface imperfections.
It is also an object of the present invention to provide a cost-effective supercalender roll with such a surface.
It is another object of the present invention to provide such a supercalender roll that capitalizes on the cost and weight advantages of filled supercalender rolls.
These and other objects of the present invention are satisfied by the present invention, which is directed to a bone-hard supercalender roll with a polymeric cover. In one embodiment, the bone-hard supercalender roll of the present invention comprises: an elongate shaft having a longitudinal axis; a core layer formed of fibrous material circumferentially covering the shaft; means for compressing the core layer along the shaft longitudinal axis; an intermediate layer circumferentially covering the core layer that comprises a first polymeric resin and a heavy textile material; and an outer layer circumferentially covering the intermediate layer that comprises a second polymeric resin and a reinforcing material. In this configuration, the roll can provide the requisite bone-hard surface for calendering applications, but can do so without the surface denting and marring problems associated with filled rolls and the expense of rolls formed of covered metal cores.
In another embodiment, the present invention is directed to a bone-hard supercalender roll comprising: an elongate shaft; an intermediate layer circumferentially covering the shaft that comprises a first polymeric resin and a heavy textile material; and an outer layer circumferentially covering the intermediate layer that comprises a second polymeric resin and a reinforcing material. In this embodiment, there is no hollow metal core (as has been the case for many prior art rolls with polymeric covers) nor its associated expense, and the intermediate layer and outer cover provide the requisite bone-hard surface for calendering.
In each of these embodiments, the inclusion of the heavy textile fiber material can occupy volume within the roll and provide structural integrity thereto without the expense of a metal core or the denting and marring problems associated with filled rolls. The heavy textile material has proven to provide a sound bonding substrate for the outer cover, and it can also bond effectively to the fibrous material of a core layer. It is preferred that the heavy textile material be a coarse fiberglass fabric; more preferably, the fabric has a mock leno weave, which provides a relatively high effective thickness to the fabric, particularly for multiple overlying plies, and also provides roughness to the fabric to improve interlaminar bonding and shear strength.